Conventionally practiced means for coating substrates such as fibrous nonwoven and paper sheet materials include knife coating, hot melt coating, aqueous dispersion coating, gravure coating, direct blade coating, roll coating, air doctor coating and squeeze coating. There are a number of problems with these known methods. Knife coating results in almost a complete surface coverage of the substrate, thereby altering its porosity and permeability properties. Only a small fraction of the permeability of the substrate is maintained mainly through the cracks that appear on the coating after drying. Hot melt coating, depending on how it is applied, may result in three-dimensional dots or islands of coating between the fibers of the substrates such that some of the pores of the sheet material are covered completely while some are partially covered.
The coating lines used in conventional methods are large and costly in terms of investment. These methods are not flexible in that a lot of coating material must be deposited, which is also economically unattractive in situations when less material would be sufficient for the end use application of the coated sheet. Solution or dispersion coatings must be dried by evaporation of the solvent or dispersion liquid in a way that the coating or the substrate are not damaged, or their properties are not altered due to exposure at high temperatures, which is a slow and costly step. In addition, in solvent-based coatings, solvent recovery is required which adds cost and complexity to the process. Coatings applied as hot melts often alter the thermal characteristics of the substrates resulting in a compromise of their original properties. Another problem is that the air permeability and/or the porosity of the uncoated substrate is almost always sacrificed once the coating is applied. It would be desirable to enhance or impart certain desired properties to a sheet material while leaving the air permeability or porosity of the material unaffected.
U.S. Pat. No. 6,083,628 (Yializis) discloses a hybrid film comprising a first polymer base film having at least one plasma-treated surface and at least a second thin acrylate polymer film disposed along the plasma-treated surface of the base film. The acrylate film is formed by crosslinking a functionalized acrylate monomer or oligomer. The hybrid film may additionally comprise one or more metallic or ceramic coatings. The continuous process for forming the hybrid film takes place within a vacuum chamber and it consists of plasma treatment of the base film to functionalize the film followed by vapor deposition of the acrylate monomer onto the base film and then radiation polymerization to crosslink the monomer. The hybrid film is useful in a number of applications including food packaging to improve barrier properties. In food packaging applications, the acrylate coating is disclosed to be typically 0.5 to 2 μm thick.
U.S. Pat. No. 4,842,893 (Yializis et al.) discloses a method for coating a flexible substrate with a thin, “substantially continuous” film by depositing a vapor of polyfunctional acrylate monomer, under vacuum, on a movable substrate maintained at a temperature such that the monomer condenses on the substrate. The film is then exposed to radiation in order to polymerize the film. The acrylate monomer coating may be formed having a thickness of less than 4 μm, preferably less than 2 μm and possibly as thin as 0.1 μm. Such coatings are disclosed as useful in food packaging and protective coatings for metal or other substrates used in a variety of applications. U.S. Pat. No. 5,032,461 (Shaw et al.) discloses a similar process in which the process of U.S. Pat. No. 4,842,893 (Yializis et al.) is repeated many times to form a multi-layered thin film structure having as many as 4,000 or more layers.
WO 99/59185 and WO 99/58756 disclose a process for coating a substrate in which the substrate is treated with a plasma and coated with an acrylate monomer, and the monomer is subsequently radiation polymerized. The plasma is generated using hollow cathodes and focused at the surface of the substrate using an electromagnetic or magnetic focusing means. According to WO 99/59185, the monomer coating may be applied using a capillary drip system, by immersion in a solution bath or by vapor deposition. According to WO 99/58756, the monomer is applied by vapor deposition. WO 99/58756 also discloses apparatus for treating industrial sized, continuous substrates, specifically such as papermaking fabrics.
A system and apparatus useful for vacuum deposition polymer coating in which a web surface is coated with inorganic and organic compositions is described in R. E. Ellwanger, M. G. Mikhael, A. Yializis and A. Boufelfel, “Vacuum Functionalization of Web Surfaces via Plasma Treatment and Polymer Coating,” Vacuum Technology & Coating (February 2001). The system includes treating the surface with plasma to remove low molecular weight material and to functionalize the surface with polar groups, depositing materials such as radiation curable acrylates onto the surface by vacuum evaporation coating and polymerization of acrylates with either an electron beam or an ultraviolet lamp. Among the applications of the coated substrates disclosed are high barrier films, printable films and nonwoven fabrics.
U.S. Pat. Nos. 5,260,095, 5,547,508 and 5,395,644 (Affinito) disclose a process and apparatus for forming solid polymer layers under vacuum, including the step of degassing the monomer material prior to injection into the vacuum. The advantages of forming polymer layers in a vacuum are said to be that photoinitiator is not needed for polymerization, polymerization is faster, there are fewer impurities in the polymer, and the polymer has a greater density and a smoother finished surface.
WO 98/18852 discloses a process for coating substrates such as polypropylene, polyester or nylon sheet materials with crosslinked acrylate and a layer of metal. Acrylate monomer is evaporated using flash evaporators and condensing the acrylate onto the sheet as a monomer film, and subsequently polymerizing the film by irradiation by electron beam or ultraviolet light. The adhesion of the acrylate on the sheet material is enhanced by plasma treatment immediately before coating. Both the plasma treatment and the coating are conducted under vacuum. The resulting coated sheet is has low oxygen permeability and is especially useful for food packaging.
U.S. Pat. Nos. 5,811,183 and 5,945,174 (Shaw et al.) disclose sheet materials including paper and film coated with acrylate polymer release coatings made by a process in which silicon-containing and fluorine-containing acrylate prepolymer having a molecular weight between 200 and 3000 is vapor deposited on the sheet material and radiation polymerized. Coating layers of between 0.5 and 1 μm thick are disclosed.
Fiber tear is an important problem with current medical packaging in which at least one fibrous sheet, such as a nonwoven or paper, and a second sheet have been heat sealed together to form a pocket capable of containing an article such as a sterile medical device. Fiber tear occurs during the opening of a package (i.e., upon peeling the two heat sealed sheets away from each other), and begins by separating a fiber or a bundle of fibers from the surface of the fibrous sheet. Fiber tear is unacceptable in the case of medical packaging because foreign particles are thereby introduced into the sterile field of the operating room. It would be desirable to eliminate the incidence of fiber tear from heat sealed packages which include at least one fibrous sheet, without greatly affecting the permeability of the fibrous sealing sheet.
Additionally, it is often necessary to provide printed information on the surface of heat sealed sheets, especially on medical packaging. However, depending upon the nature of the substrate used as the heat sealed sheet, such printing can be rubbed off of the surface rather easily. It would be desirable to provide a coating on such substrates which would enhance ink adhesion to the heat sealed sheet/package.
Additionally, many sheet materials which are used in protective environments, such as in the medical field for use in making gowns, masks, drapes, boots, etc., are subject to contact with fluids which may present either chemical or biological hazards. Accordingly, such materials are typically rated as to their resistance to fluid strike-through, particularly blood strike-through in the medical field. It would be desirable to provide coatings for such sheet materials which would enhance fluid strike-through resistance without significantly affecting the air permeability of the sheet, such that the wearer will enjoy increased protection from fluid strike-through while still being comfortable when wearing such garments.